A contact IC card which incorporates a semiconductor integrated circuit having CPU (Central Processing Unit) and a function like a memory, and which has a contact terminal of the semiconductor integrated circuit on a surface thereof has been in common use in finance and other fields.
Such contact IC card is managed by CPU or the like in write on and erase from a memory and has e.g. a cipher processing function, which actualizes the high security performance of the contact IC card. In regard to a device such as CPU, which realizes the function like this, the breakdown voltage has been lowering owing to the scale-down of semiconductor processes in recent years, and therefore a source voltage supplied to CPU is restricted to a level which never exceeds the breakdown voltage of the device. For this purpose, it is common to supply a source voltage to CPU through a regulator for restricting the voltage level of a source-voltage terminal.
However, a noncontact IC card, which does not have a power source such as a battery and which operates producing a source voltage for allowing an internal circuit to work from electromagnetic waves received through an antenna, has been used in the fields of transport, etc. This type of noncontact IC card receives input data, which have been transmitted by means of modulated electromagnetic waves from a reader/writer (interrogator), performs a signal processing of input data thus received to produce output data, and modulates electromagnetic waves by use of a load between antenna terminals varying according to the output data to transmit output data to a reader/writer (interrogator).
Like a contact IC card, a noncontact IC card has CPU, a memory, etc. therein provided for achieving the functions as described above. Therefore, it requires that CPU and other parts should be supplied with source voltages restricted so as not to exceed the breakdown voltages of the elemental devices.
The patent document, U.S. Pat. No. 7,505,794 discloses a method for solving the problem that as to a series regulator incorporated in a noncontact IC card, a compensation current for load variation deteriorates the quality of communication with a reader/writer, whereas a current of a shunt regulator provided in the noncontact IC card counterbalances the variation of current of a load-modulation circuit. According to the method described in U.S. Pat. No. 7,505,794, the series regulator operates and the shunt regulator stops in case that the noncontact IC card transmits a signal to a reader/writer, whereas the series regulator stops and the shunt regulator works except in case that a signal is transmitted to the reader/writer.
A dual-way IC card having both the function of a contact IC card and the function of a noncontact IC card is becoming popular. In a dual-way IC card, either a source voltage supplied through a regulator from a source-voltage terminal, which is a contact terminal, or a source voltage produced from electromagnetic waves received through an antenna is selected according to its working condition. The selected source voltage is supplied to an internal circuit of e.g. CPU incorporated in the dual-way IC card. The electric power supplied through the source-voltage terminal or antenna allows the internal circuit of the dual-way IC card to have both the function of a contact IC card and the function of a noncontact IC card.
On the other hand, the patent document, Japanese Unexamined Patent Publication No. JP-A-2000-113148 discloses a method for solving the problem that in a combination card having the function of a contact IC card and the function of a noncontact IC card, electric power leaks out from a source terminal serving as a contact terminal used in a contact mode because the contact terminal is put in conduction in a noncontact mode. Specifically, the problem is that under the condition that a leakage from the contact terminal takes place in the noncontact mode, if a voltage higher than that of the inside of IC chip is applied to the source terminal during operation in the noncontact mode, IC chip will be under an electrical attack. Therefore, the method described in JP-A-2000-113148 includes: connecting a signal switch between the contact terminal and an internal circuit of the combination card; connecting a power-on switch between the contact terminal used as a source terminal and the internal circuit of the combination card; and keeping off the signal switch and power-on switch in a noncontact mode operation. The arrangement like this makes it possible to electrically isolate all the contact terminals from the internal circuit of the combination card during operation in the noncontact mode.